1. Field of the Invention
The present invention relates to an encoder that detects a relative displacement between an encoder head and an encoder scale having a pattern of a prescribed cycle.
2. Description of the Related Art
Encoders are devices, each designed to generate a plurality of cyclic signals that are different in phase. The cyclic signals output from the encoder are supplied to a processing circuit which measures the cycle of each signal. From the cycles measured, the processing circuit can determine, for example, the moving direction of the object having an encoder scale is moving, the position of the object takes, the displacement of the object, and the displacement speed of the object.
An encoder configured to detect the displacement a relative displacement between an encoder head an encoder scale having a prescribed cyclic optical pattern is disclosed in, for example, Jpn. Pat. Appln. No. 6-26817. In the encoder, the light emitted from the light source mounted on the encoder head is applied to the encode scale that is moving relative to the encoder head. The light reflected, diffracted or scattered at the encoder scale is guided to a plurality of light receiving elements provided on the encoder head. Assume that the light is guided to two light receiving elements arranged with a phase difference of 180 between them. Then, the photoelectric currents the light receiving elements have generated are input to the current-to-voltage conversion circuit incorporated in the encoder. The current-to-voltage conversion circuit converts the photoelectric currents into voltage signals VPA and VPAB, respectively. Each of the voltage signals VPA and VPAB contains not only the AC component representing the change in the intensity of light coming from the encoder scale, but also the DC component representing the fixed intensity of the light and the noise contained in the voltage signal. The noises contained in the voltage signals VPA and VPAB, respectively, are equal to each other. In order to remove the DC components and the noises from the voltage signals VPA and VPAB, the subtraction circuit provided in the encoder performs an operation of: VREF−(VPAB−VPA), where VREF is a reference voltage. The subtraction circuit generates a cyclic signal VA that represents the change in light intensity only. The reference voltage VREF is a fixed voltage that has been generated by dividing the power supply voltage VCC. The reference voltage VREF is applied to the current-to-voltage conversion circuit and subtraction circuit. The reference voltage VREF is used as the reference level of cyclic signals.
The encoder has a plurality of units, each composed two light receiving elements and a signal processing circuit. These units generate cyclic signals of different phases, which are used as an encoder signal. The encoder can divide the phases of two cyclic signals that differ in phase by, for example, 90°, each into tens to thousands of segments. This enables the encoder to detect, at a higher precision, the relative displacement of the encoder head and the encoder scale.
Hitherto, the AC component and the DC component are supplied to the two current-to-voltage conversion circuits associated with the two light receiving elements and are converted to voltage signals. Thereafter, the subtraction circuit subtracts one of the two voltage signals differing in phase by 180° from the other voltage signal, thereby eliminating the DC component contained in the same amount in the two voltage signals. To obtain the encoder signal that contains a little noise and greatly changes in voltage, each current-to-voltage conversion circuit should better generate a sufficiently large voltage signal and the subtracting circuit should not amplify the signal so much.
In order to miniaturize the encoder, reduce the price of the encoder and lower the power supply voltage of the encoder, the light source, the light receiving elements and a IC comprising the light source driver and signal processing circuit may be encapsulated in a transparent resin mass and mounted on the encoder head (the light receiving elements may be incorporated in the IC.) If the encoder is so configured, however, an DC component resulting from the light reflected and scattered in the resin mass and not changing in magnitude, irrespective of the relative displacement of the light receiving elements and the encoder scale, will be more generated than hitherto, in addition to the AC component and DC component that has resulted from the light reflected, diffracted or scattered at the encoder scale. In such a conventional encoder, the DC component not contributing to the encoder signal is so large that the current signal corresponding to the AC component generated by each light receiving element cannot be converted into a voltage signal having sufficient amplitude by the current-to-voltage conversion circuit, which should be supplied to the next-stage circuit. Further, the voltage signal may have a voltage falling outside the output range of the current-to-voltage conversion circuit, and no signals may be supplied to the next-state circuit. These problems are particularly prominent if the power supply voltage VCC is low. Moreover, such the encoder signal has an insufficient amplitude or an insufficient signal-to-noise ratio even if its phase angle is determined from its amplitude and then multiplied in accordance with the phase angle. Inevitably, the encoder cannot detect the displacement at high precision.
The problem resulting from the low power supply voltage arises not only in reflection-type encoders, but also in transmission-type encoders.